1. Field of the Invention
The present invention relates to a charge director composition for liquid toner formulations.
2. Description of the Prior Art
Liquid toner compositions are used in office copy machines, computer print-out devices, lithographic master preparation and the like to create a visible counterpart from a latent electrostatic image. Liquid toners generally consist of five components: a carrier liquid, coloring agent, fixative agent, dispersing agent and charge director. In any given toner composition, thre may be one or more of each of these components. Also, one or more chemicals in such toner compositions may simultaneously have multiple functions. For example, a dispersing agent may also act as a fixative. Moreover, when a polymeric dispersing agent is employed, the combination of coloring agent, fixing agent and dispersing agent is sometimes called a dyed latex solid toner polymer.
A carrier liquid component for a liquid toner composition must have a low specific conductivity (e.g. resistivity of greater than 10.sup.10 ohms cm), a low dielectric constant (e.g. less than 3.5), a low viscosity and a rapid evaporation rate. Furthermore, such a carrier liquid should also preferably have low toxicity, low cost, poor solvent power, no odors, chemical stability and a high flash point. With all of these restrictions together, the preferred choice is an aliphatic hydrocarbons, most preferably an odorless mineral spirit in the TCC flash point range of 101.degree. to 150.degree. F. Isopar G or H solvents made by Exxon Corporation are typical of particularly preferred aliphatic hydrocarbons.
In the development of the electrostatic latent image to a visible image, the coloring agent or solid particles (including dyes or pigments) in the toner composition either migrate to the charged areas or the uncharged areas but not to both. If the coloring agent or solid particles go to the charged areas, this is called positive development. If the particles go to the uncharged areas, this is called reversal development. The coloring agent should be essentially insoluble in the carrier liquid and preferably contain no contaminants which are soluble therein. Dyes are selected from their solubility in the fixing agent and insolubility in the carrier liquid as well as their color. Moreover, pigments are chosen on the basis of proper color, the best intrinsic surface or migration properties, the ease of grinding the coloring agent to a desired fine particle size, and the smallest differential between the specific gravities of the pigment and the carrier liquid. Both dyes and pigments should preferably be chemically stable and light-fast.
In order to create a stable dispersion of the pigment particles in the liquid carrier, a dispersing agent is normally used. Generally, this stable dispersion is made by grinding a slurry of the pigment particles in the carrier liquid in the presence of a sufficient amount of the dispersing agent or agents. Most commercial dispersing agents are surface-active molecules (i.e. they possess a polar end and a non-polar end). It is believed that the polar end part of the molecule is absorbed on the surface of the pigment molecule while the non-polar end is oriented away from that particular surface into the surrounding liquid carrier phase. Thus, a dispersing agent is preferably chemically stable, soluble in the liquid carrier continuous phase and absorbable by the pigment particles.
In contrast, dyes are usually employed in dyed latex solid toner polymers. Accordingly, the dyes are incorporated therein by reacting them into the polymer or by dissolving them into a swelled solid latex polymer particle.
The fixative agent aids in the making of the toned or visual image a permanent part of the underlying substrate (e.g. paper). These fixative agents are generally natural resins or synthetic polymers which have the desirable characteristics of chemical stability, an unobjectable color, and may be preferably insoluble in the liquid carrier as well as be compatible with a substrate onto which the image is deposited. There are many commercially available resins useful for this purpose.
The last component of a liquid toner is the charge director. The charge directors must be soluble or dispersible in the hydrocarbon liquid carrier and must create or augment an electrostatic charge on micron or sub-micron fixative agent particles. The patent literature is replete with different charge director compositions. The majority are metal salts of long chain fatty acids, both substituted and unsubstituted.
In U.S. Pat. Nos. 3,753,760; 3,900,412; 3,990,980; and 3,991,266, all of which issued to Kosel and are each incorporated herein by reference in their entirety, teach the creation of a multi-functional amphipathic or latex molecule which combines in one molecule the functions of colorant agent, the dispersing agent, and the fixative agent. Thus, liquid latex toners as these are sometimes called, have only three components: the carrier liquid, the multi-functional latex particle and the charge director.
One known commerically used charged director is ASA-3 antistatic additive for liquid hydrocarbons. This additive is comprised of 1-10 parts each of:
1. a chromium salt of a C.sub.14-18 alkyl salicyclic acid;
2. a calcium didecyl sulfosuccinate; and
3. a salt of the didecyl ester of sulfosuccinate acid and at least 50% of the basic nitrogen radicals of a copolymer of lauryl methacrylate, stearyl methacrylate and 2-methyl-5-vinyl pyridine (also called 5-vinyl-2-picoline) said copolymer having a vinyl pyridine content of 20-30% by weight and an average molecular weight of 15,000-250,000.
A preparation of this additive is shown in U.S. Pat. Nos. 3,210,169 and 3,380,970 (both assigned to Shell Oil Co.), both of which are incorporated herein by reference in their entireties.
This ASA-3 charge director has worked very effectively in many latex-based liquid toner compositions. However, liquid toner formulations containing this charge director composition do suffer from a gradual increase of resistivity (i.e. loss of conductance) over a period of time. This resistivity increase is a serious problem when quantities of the liquid toner containing this charge director must be stored for long periods of time, causing possible functional problems with plate or print quality.
Accordingly, there is a need in this art to improve the conductance stability of liquid toners employing ASA-3 as a charge director without adversely effecting the other desired properties of the toner formulation. The present invention is a solution to this need.